Gardening glossary
Blossom end rot
Blossom end rot is the single most common tomato disorder in home gardens — and one of the most misunderstood. The brown-black sunken patch at the bottom of an otherwise-perfect fruit looks like a disease, but it is not. No fungus, bacterium, or insect is involved. It is a physiological problem caused by the fruit failing to get enough calcium at a critical stage of development.
The mechanism is subtle. Calcium moves through the plant only with the [transpiration](/glossary/transpiration) stream — water pulled up through the xylem and out through the leaves. The actively-growing fruit, which transpires far less than a leaf, receives only a small share of that flow. If water uptake becomes irregular — a drought followed by a deluge, or a hot windy spell when roots cannot keep up — calcium delivery to the developing fruit drops below the threshold the tissue needs, and the cells at the bottom of the fruit (furthest from the stem) die.
Crucially, the soil is almost never short of calcium. University Extension publications consistently emphasise that calcium sprays and soil amendments rarely fix BER, because the problem is internal calcium distribution, not external supply.
What actually prevents BER:
1. **Even moisture.** This is the number-one factor. Mulch heavily with 5–10 cm of compost, leaf mould, or straw, and water deeply and consistently — 25 mm per week from rainfall or irrigation in summer. 2. **Healthy root system.** Plants with restricted roots (small containers, compacted soil) struggle with calcium uptake even in well-watered conditions. Use generously-sized containers and loose soil. 3. **Avoid excess nitrogen.** High-nitrogen feeding promotes rapid leafy growth that competes with fruit for calcium. Switch to a potassium-leaning feed once flowering begins. 4. **Soil pH 6.0–6.8.** Calcium is most available in this range; very acid soils can be limed with dolomitic limestone in the autumn before planting. 5. **Steady environment.** Avoid letting containers blow over, sit on hot concrete, or dry out severely between waterings.
Plants and varieties most affected:
- **Roma and plum-type tomatoes** — most susceptible. - **Container-grown tomatoes** — far more prone than in-ground. - **Peppers, courgettes, squash, watermelon, and aubergine** — same disorder, same prevention.
The earliest one or two fruit on a tomato truss are often affected because the root system is still developing. Pick the affected fruit, mulch, settle the watering rhythm, and subsequent trusses usually come through clean.